There are technologies that, in a full-text search on a file group, use a full-text search index indicating the character information making up the files included in the file group, to narrow down search-target files from the file group. By referring to the full-text search index, files that do not include the character information of a search character string are excluded from the search targets, and the search-target files are thereby narrowed down.
In certain technologies, a full-text search is carried out with respect to a file group that has been written in a markup language. The files included in the file group written in the markup language are logically (or physically) divided with tags serving as boundaries, and a full-text search index that indicates the character information making up each data unit obtained by this dividing is created. Furthermore, when a search character string and a tag are input, data units that do not correspond to the input tag and data units that do not include the character information of the search character string are excluded from the search targets, and search-target data units are thereby narrowed down (for example, refer to Japanese Laid-open Patent Publication No. 8-329116 or Japanese Laid-open Patent Publication No. 8-147311).